The present invention relates generally to telecommunications and, more particularly, to methods and apparatus for echo and feedback suppression in full duplex speakerphone type voice communication.
A speakerphone substitutes a microphone and an audio speaker, located either in a separate enclosure or included in the telephone itself, for the customary telephone handset. Some of the advantages provided by speakerphones are hands free operation, group conference calls, and being able to do other things while waiting "on hold." Speakerphones are essential for practical videoconferencing. Similar voice communication systems are also used independently of the telephone system. For example, such voice communication systems are used in intercoms of the type used for taking drive-up orders at fast food restaurants.
Since the microphone and speaker in a speakerphone are at greater distances from the user than in an customary telephone handset, amplification, in addition to that provided for the customary telephone handset, must be applied to both the signals received by and transmitted from a speakerphone. Acoustic coupling between the speaker and the microphone in a speakerphone, i.e. room echo, and electrical coupling between the signals receive by and transmitted from a speakerphone, unless properly controlled cause echoes and "feedback" or "singing."
There are two generally recognized techniques for suppressing echoes and singing in speakerphones. One technique, is known as "voice-switching" or "simplex switching." A voice-switching speakerphone includes two separate variable gain amplifiers, one amplifier being used to amplify or attenuate the signal received by the speakerphone, and the other one amplifying or attenuating the signal transmitted from the speakerphone.
In voice-switching speakerphones, a control circuit monitors the respective strength of the received and transmitted signals. If the transmit signal is stronger, then the control circuit causes the transmitted signal to receive maximum amplification while the received signal receives maximum attenuation, and conversely. To prevent singing under all circumstances, the received signal and transmitted signal amplifiers must provide a total gain between the speakerphone's microphone and its speaker that is less than one (1.0).
A significant difficulty with voice-switching speakerphones arises from their simplex operation. Simplex operation allows only one party to the telephone call to speak at a time. If both parties attempt to speak simultaneously, only one party's voice will be successfully transmitted to the other party, and perhaps neither party will hear the other. Furthermore, the time required to adjust the respective amplification and attenuation of the transmitting and receiving amplifiers in a voice-switching speakerphone momentarily clips the beginning of each message as the two parties converse back and forth.
A second generally recognized technique for suppressing echoes and singing in speakerphones is the use of adaptive Finite Impulse Response ("FIR") filters. By using a pair of adaptive FIR filters in a speakerphone, the voice of one party may be transmitted while the other party's voice is being received. Consequently, a speakerphone employing a pair of adaptive FIR filters does not clip the beginning of each message as the two parties converse back and forth, thereby allowing both parties to speak simultaneously.
In an adaptive FIR filter speakerphone, one of the adaptive FIR filters transmits an echo canceling signal from the output of a received signal summing junction to a transmitted signal summing junction in which it is combined with the speakerphone's microphone output signal. The output signal from the transmitted signal summing junction is the signal transmitted to the telephone line from the speakerphone. Simultaneously with combining an echo canceling signal with the microphone output signal in the transmitted signal summing junction, the other adaptive FIR filter transmits an echo canceling signal from the output of the transmitted signal summing junction to the received signal summing junction in which it is combined with the received signal. The output signal from the received signal summing junction is supplied to the speaker of the speakerphone.
To permit a digital signal processing implementation of adaptive FIR digital filters, both the signal received by the speakerphone and the signal produced by its microphone are first digitized. These digital signals are then processed by digital circuits which provide the adaptive FIR filters and the summing junctions. The digital signals resulting from this digital signal processing are then converted back to analogue signals for application to the speakerphone's speaker and for transmission to the telephone line.
To convert between analogue and digital signals, an adaptive FIR digital filter speakerphone includes both a pair of Analogue to Digital Converters (ADCs"), and a pair of Digital to Analogue Converters ("DACs"). One of the ADCs converts the received analogue signal into a digital signal for processing by the adaptive FIR digital filter while one of the DACs converts the processed digital received signal back to an analogue signal. Similarly, the second ADC converts the analogue signal from the microphone into a digital signal for processing while the second DAC converts the digital transmitted signal back to an analogue signal. In addition to the pair of adaptive FIR digital filters, ADCs, and DAC's, an adaptive FIR digital filter speakerphone also includes a pair of variable gain amplifiers for respectively increasing the strength of the received and transmitted signals.
The use of digital signal processing techniques, including adaptive FIR digital filters, for echo canceling in the simultaneous reception and transmission of telephone signals is described in chapter 15 of a book entitled "Digital Signal Processing Applications with the TMS320 Family--Theory, Algorithms, and Implementation," copyright 1986, Texas Instruments Incorporated. Chapter 15, entitled "Digital Voice Echo Canceller with a TMS32020," specifically describes using a Texas Instruments TMS32020 digital signal processor integrated circuit to construct a digital echo canceler which meets or exceeds the performance required under CCITT G.165 recommendations.
While adaptive FIR digital speakerphones suppress echoes and feedback to the extent required for successful operation, there still remains some residual echo due to feedback from the speaker to the microphone that is not canceled by the adaptive FIR filter.
Another problem with the adaptive FIR digital filter speakerphone arises from the wide dynamic range of the received and transmitted signals. To compensate for this wide dynamic range, adaptive FIR digital filter speakerphones generally apply Automatic Gain Control ("AGC") to both the transmitted and received signals. However, during periods of silence, when neither party, is speaking, the low signal level causes the AGC circuit to increase the amplifiers' gain to such an extent that background room noise dominates.
To remove the residual portion of echoes during intervals of low signal level, prior speakerphones may perform what is known as "echo suppression" by either disabling the corresponding output signal or by substituting white noise for the corresponding output signal. However, in typical conference rooms or offices, this switching is apparent and it is obvious that the incoming signal is being switched off in response to signal level changes.